In an optical communication system, information is transmitted by using optical signals of different modulation formats as carriers. Modulation format is mainly used for carrying and transmitting information. Requirement for modulation formats mainly aims at fast transmission rate and good signal quality, wherein the indicator for measuring the transmission rate is spectral efficiency and the indicator for measuring the transmission quality is power efficiency. In order to transmit signals in longer distance with faster rate and greater capacity, it is an important research content to continuously improve modulation formats of transmission signals. In order to increase transmission capacity, high order modulation format is generally used, for example, Quadrature Phase-Shift Keying (QPSK) and 16-ary Quadrature Amplitude Modulation (16QAM), etc. Signals of these modulation formats can transmit multiple bits of information at each sign bit. In order to increase transmission distance, signals are required to have better power efficiency, for example, M-ary Pulse Position Modulation (mPPM) signals. In 2011,polarization-division-multiplexed mPQ modulation (mPPM-QPSK, combined modulation of M-ary Pulse Position Modulation and Quadrature Phase-Shift Keying) signal was proposed by Bell Laboratory, which initially achieves a transmission system with large capacity and high power efficiency. However, compared to QPSK, this method sacrifices a large amount of transmission bandwidth while achieving higher power efficiency, which directly results in a decrease in spectral efficiency of the transmission signal (spectral efficiency mainly refers to bit rate of transmission/bandwidth occupied by optical signals).
Specifically, the above-mentioned mPQ modulation format is a combined modulation of M-ary Pulse Position Modulation on basis of polarization-division-multiplexed QPSK/QAM signals. Each original sign bit is divided into m signal slots, wherein one of the signal slots is loaded with the information in the original sign bit, and the time domain width of each signal slot is only 1/m of the time domain width of each original sign bit, and thus, the bandwidth in frequency domain occupied by the signal is m times of that of the original signal. Furthermore, the ability of mPQ modulation format to improve the bit rate of signal transmission is poor, such that signal spectral efficiency (bit rate of transmission/bandwidth occupied by optical signals) is significantly decreased. Under the condition that hardware equipment of the entire set transmission system is unchanged, transmission signal cannot exceed the maximum signal bandwidth which an electric equipment can meet, and then an mPQ signal with poorer spectral efficiency can only achieve lower transmission rate, which restricts transmission capacity.
Thus, how to decrease spectral efficiency loss or even eliminate spectral efficiency loss in optical communication while improving power efficiency is an urgent problem to be solved.